In the majority of electronic systems, speed is a fundamental parameter among the parameters that determine the global performance of an electronic system. As far as the so-called “system on chip” is concerned, the handling of off-chip electrical signals is more critical and poses more problems than the handling of on-chip signals.
The need of reliably achieving very high communication frequencies has led to the abandonment of CMOS full-swing signals (that is, signals that vary from the negative voltage supply Vss to the positive voltage supply Vdd). This was motivated by the difficulty of outputting signals of extremely high frequency, especially when they are to be conveyed over a long conduction line of a printed circuit board and/or over a long cable having a low matching impedance.
The step forward was to produce differential output signals onto matched lines, using a reduced standard output swing of only 350 mV per signal. Thus, it has become possible to transfer data at enhanced speeds, reducing interferences and power consumption, and at the same time, improving common mode noise rejection. An example of this technique is the standard low voltage differential signals (LVDS). FIG. 1 depicts a basic diagram of an LVDS standard cell, the functioning of which is well known.
Upon increasing the frequency beyond 1 GHz another problem becomes relevant. Because of the skin effect in conductors, the resistance thereof increases according to a non-linear law as a function of the frequency. This causes a non-linear attenuation as the frequency increases.
In addition, manufacturers of telecommunication systems often tend to continue to use old design boards, updating only the electronic components and/or to employ low quality materials for lowering the cost of printed circuit boards. This may cause significant attenuations due to poor dielectrics as the frequency increases.
To alleviate these degrading effects, two techniques have been developed that may be used alone or in combination with each other. These two techniques are adaptive equalization of the line and pre-emphasis of the signal to be transmitted.
The first technique uses a stage in which the gain varies with frequency to compensate for attenuation along the transmission line. The article by J. Y. Sim et al. “A CMOS Transceiver For DRAM Bus System With A Demultiplexed Equalization Scheme”, IEEE J. Solid-State Circuits, vol. 37, pp. 245-250, February 2002, describes an equalized transceiver that uses a particular equalization system for reducing inter-symbolic interference. Adaptive equalization, besides requiring more complex circuits, reduces the signal/noise ratio.
In contrast, the pre-emphasis technique varies the spectral content of the transmitted signal to obtain a transfer function of the cascade of the pre-emphasis network and of the transmission line that is almost constant with the frequency in the band of interest. When the line attenuation is relatively small, a pre-emphasis amplification is sufficient only during or even immediately after the switching transients of the signals to be output.
U.S. Pat. No. 6,288,581 to Wong and U.S. Pat. No. 6,281,715 to DeClue et al. disclose LVDS drivers with pre-emphasis. These circuits amplify a digital signal to be transmitted with an enhanced gain coinciding with the switching of the signal compared to the gain during the phases in which the signal maintains a constant value.
In particular, the '581 patent discloses a LVDS driver with pre-emphasis having two standard LVDS cells, as shown in FIG. 2. The LVDS driver includes enabling transistors 44, 54 and 24, 34 and the output nodes of which are connected through switches 60 and 62. When the signal to be transmitted remains constant, the output differential signal VOP, VON is generated only by one LVDS cell. When the signal represented by the differential pair V+, V− is applied to transmit the switches, the switches 60 and 62 are closed and remain in a conduction state as long as the transient lasts. This results in the two LVDS cells generating the output differential signal VOP, VON.
This technique has the drawback of requiring a good synchronization of the turning on of switches 60 and 62 with the switching edges of the signal to be transmitted. This is complicated because of the turn-on delays of the switches.
The '715 patent discloses a single stage driver with pre-emphasis having two current mirrors, wherein one always biases the output stage while the other is operatively connected only when the relative control circuit detects a switching of the signal to be transmitted. In this second case, the problem is to synchronize the turning on of the second current mirror with the switching edges of the signal to be transmitted. Consequently, this limits the transmission speed.